In many fields of technology the transmission of speech is carried out in analog form, i.e. via an analog transmission channel, which is also referred to as speech or voice channel. One example is the radio communication between the crew onboard aircraft and ground personnel, e.g. between the pilots and the tower. Such radio communication may be effected, for example, using the HF frequency range in order to allow for long range communication.
It is often desirable to also transmit data in addition to speech. Although this can be done by separate means and via a separate channel, it is potentially more efficient and cost effective to transmit the speech and the data over the same channel. However, due to the limited band width of the speech channels used for transmission of speech, it is problematic to achieve a reasonably high data rate without compromising the quality of the speech transmission.